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Langmuir 1995,11, 1852-1854
Conformational Rigidity in a Self-AssembledMonolayer of 4-Mercaptobenzoic Acid on Gold Stephen E. Creager" and Christine M. Steiger Department of Chemistry, Indiana University, Bloomington, Indiana 47405 Received December 21, 1994. I n Final Form: March 8, 1995@ Self-assembledmonolayers of 4-mercaptobenzoic acid exhibit infrared spectral features for the carboxylic acid functionalitythat are characteristic of isolated vapor-phase molecules (e.g.,sharp 0-H stretch band, high-frequency non-hydrogen-bonded carbonyl stretch band) and inconsistent with molecules that are dimerized or that reside in condensed-phase hydrogen-bonding environments. This is in sharp contrast with the behavior of o-mercaptoalkanoic acid monolayers, which exhibit spectral features characteristic of hydrogen-bonded and/or dimerized acid groups (e.g., no obvious 0-H stretch band, lower frequency carbonyl stretch band). It is postulated that these characteristic differences reflect differing degrees of conformational rigidity in the monolayers, with the aromatic monolayers being sufficiently rigid and oriented as to prevent the intermolecular hydrogen bondingldimerization that is commonly observed in aliphatic monolayers. Molecular self-assembly methods are commonly used to prepare organized molecular assemblies on s~rfaces.l-~ Such assemblies can serve as excellent model systems for studying a variety of interfacial processes, including electron, proton, and energy transfer, wetting, and friction, and they should find use in studies of bioaffinity and biocompatability of surfaces, and in the preparation of biological sensors. A great majority of the research effort on such systems has focused on functionalized alkanes, often alkanethiols, on metal surfaces, often noble metals such as gold. Dispersive forces among the extended alkyl chains in a nearly close-packed monolayer are thought to contribute to the stability and rigidity (relative to a free alkane) of the resulting monolayers. Such monolayers are not completely rigid, however, as evidenced by a small but nonzero population of gauche bonds that have been shown by Raman spectroscopy to be present in the nominally extended alkyl chains4 and by the fact that functional groups such as carboxylica~ids,~-lO amines, and alcoholsll that are capable of forming intermolecular hydrogenbonded aggregates inevitably do so when they are present at the terminal position of alkanethiols that comprise the monolayers. .This conformational flexibility can be a problem in applications that depend heavily on the enforcement of a specific spacing and/or conformation among components in a monolayer. It is partially for this reason that structures based on more rigid molecular building blocks, for example polyphenylenes,12oligomeric [l.1.llpropellane-basedstructures (~taffanes),'~ norbornylAbstract published in Advance ACS Abstracts, May 1, 1995. (1) Ulman, A. An Introduction to Ultrathin Organic films. From Langmuir-Blodgett to SelfAssembly; Academic Press, Inc.: San Diego, CA, 1991. (2)Whitesides, G. M.; Laibinis, P. E. Langmuir 1990,6, 87. (3)Dubois, L. H.; Nuzzo, R. G.Annu. Rev. Phys. Chem. 1992,43,437. (4)Bryant, M. A.;Pemberton, J. E. J . Am. Chem. SOC. 1991,113, 8284. (5) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J.Am. Chem. SOC. 1990, 112,558. (6)Chidsey, C. E. D.; Loiacono, D. N. Langmuir 1990,6, 682. (7)Duevel, R. C.; Corn, R. M. Anal. Chem. 1992,64,337. (8)Sun, L.; Kepley, L. J.; Crooks, R. M. Langmuir 1992,8, 2101. (9)Sun, L.; Crooks, R. M.; Ricco, A. J. Langmuir 1993,9, 1775. (10)Zhang, M.;Anderson, M. R. Langmuir 1994,10,2807. (11) Sprik, M.; Delmarche, E.; Michel, B.; Rothlisberger, U.; Klein, M. L.; Wolf, H.; Ringsdorf, H. Langmuir 1994,10,4116. (12)Sabatani, E.; Cohen-Boulakia, J.; Bruening, M.; Rubinstein, I. Langmuir 1993,9, 2974. (13)Obeng,Y. S.;Laing,M. E.;Friedli,A. C.;Yang,H. C.; Wang,D.; Thulstrup, E. W.; Bard, A. J.; Michl, J. J. Am. Chem. SOC.1992,114, 9943. @
based structures,14 and oligopeptides15J6have been investigated. Such structures are of particular interest in studies of long-range electron transfer, which is exquisitely sensitive to distance and which should therefore be strongly affected by slight variations in the distance between a molecular donor/acceptor and an electrode. Applications of rigid monolayers are also envisioned in the study of molecular nonlinear optical effects at surfaces, which typically occur only in rigid molecular assemblies that can force an ensemble of dipolar molecules to adopt a specific orientation relative to the surface and then maintain that orientation for very long periods of time. The central finding of the present work is that an organized monolayer based on an aromatic motif, specifically a self-assembled monolayer of 4-mercaptobenzoic acid on gold, completely inhibits the formation ofhydrogenbonded dimers and/or aggregates of carboxylicacid groups that are routinely formed in similar monolayers based on alkanoic acids. This finding is supported by reflectance infrared spectroscopy of self-assembled monolayers of 4-mercaptobenzoic acid and 16-mercaptohexadecanoic acid on gold. Monolayers were formed on freshly prepared gold mirrors by immersion in 1 mM solutions of the appropriate thiol compounds in absolute ethanol for not less than 24 h. 4-Mercaptobenzoic acid was purchased from Toronto Research Chemicals, and 16-mercaptohexadecanoic acid was prepared via a literature method.6 Gold mirrors were prepared by vacuum evaporation of 2000 A of gold onto (3-mercaptopropy1)trimethoxysilane-treated glass microscope ~1ides.l~ Reflectance spectra were measured using a Nicolet Magna 550 Fourier-transform infrared spectrometer fitted with a Harrick Seagull specular reflectance accessory. Light was incident to the surface at 80" relative to the surface normal and was p-polarized. Figure 1 presents reflectance spectra in the hydrogen region (3700-2400 cm-l) for monolayers of 16-mercaptohexadecanoic acid (top) and 4-mercaptobenzoic acid (bottom). The spectrum for the aliphatic monolayer exhibits the expected features at 2849 and 2918 cm-l for the symmetric and asymmetric methylene stretching vibrations in the alkyl chain but no features for the acid (14)Black, A.J.;Wooster, T. T.; Geiger, W. E.; Paddon-Row, M. N. J.Am. Chem. SOC. 1993,115,7924.
(15)Enriquez,E. P.;Gray,K.H.;Guarisco,V.F.;Linton,R. W.;Mar, K. D.; Samulski, E. T. J. Vue. Sci. Technol. A 1992,10,2775. (16)Whitesell, J. K.; Chang, H. K. Science 1993,261,73. (17)Goss, C. A,; Charych, D. H.; Majda, M. Anal. Chem. 1991,63, 85.
0743-746319512411-1852$09.00/0 0 1995 American Chemical Society
Letters
Langmuir, Vol. 11, No. 6, 1995 1853 0.0014.
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Figure 1. Infrared reflection-absorption spectra in the hydrogen region for monolayers of 16-mercaptohexadecanoic
acid (top) and 4-mercaptobenzoicacid (bottom)on gold mirrors. Details of monolayer preparation and spectral acquisition are in the text.
2200
2000
18W
lsoo Wsvenumberr lcm-11
Figure 2. Infrared reflection-absorption spectra in the fingerprintregion for monolayers of 16-mercaptohexadecanoic acid (top) and 4-mercaptobenzoic acid (bottom)on gold mirrors.
groups, in agreement with published ~pectra.~-l* The reflectance spectrum of the aromatic monolayer lacks any strong features corresponding to C-H stretches; however it does exhibit a prominent, sharp peak at a very high frequency of 3579 cm-l. This peak undoubtedly corresponds to the free O-H stretch that is commonly observed in the high-temperature vapor-phase spectra of carboxylic acids (for example, at 3581 cm-' for vapor-phase benzoic acid at 290 OC18)but is almost never observed in condensed phases due to hydrogen bonding andlor dimerization of the acid groups. The absence of this peak in the spectra of aliphatic monolayers has been taken by others as evidence of hydrogen bonding andlor dimerization of acid groups in such monolayers. It is remarkable that the use of an aromatic framework in place of an aliphatic one completely inhibits formation of these aggregates. Figure 2 provides further evidence that acid groups in the aromatic monolayer are in a free, unaggregated state. The aliphatic monolayer exhibits a spectral feature for the carbonyl stretch that appears to be comprised of two peaks, a major one at 1717 cm-l with a shoulder at 1740 cm-l. The peak at 1717 cm-l is characteristic of a carboxylicacid in a hydrogen-bonding environment, again as noted in several published work^.^-'^ In contrast, the spectrum of the aromatic monolayer exhibits a single peak at a comparatively high frequency of 1749cm-l. Carbonyl stretching frequencies in this range are characteristic of acid groups that are unassociated; for example, the carbonyl stretch peak appears at 1768 cm-' for benzoic acid in the vapor phase and a t 1688 cm-l for unassociated benzoic acid in a Nujol mu11.18 In fact, the presence of the
high-frequency shoulder in the carbonyl stretch band for the aliphatic monolayer has been taken as evidence for the presence of a small population of non-hydrogen-bonded acid groups in these monolayers. Apparently, the aromatic monolayer adopts this structure exclusively. Table 1presents a summary of the peak positions and assignments for the spectral features in Figures 1and 2 and for spectra of the corresponding compounds in KBr glass. The presence of the free OH stretch and the high frequency of the carbonyl stretch in the aromatic monolayer confirm that the acid group is unassociated, as discussed above. Some other features of these spectra also bear comment. The absence of any strong features associated with aromatic C-H stretches in the monolayer spectrum may be an orientation effect, suggesting that the transition dipoles for these transitions are oriented largely parallel to the gold surface. Alternatively, these transitions may be symmetry forbidden or simply very weak, as suggested by the fact that the vapor-phase infrared spectra of many para-substituted benzoic acids (for example, 4-fluorobenzoic acid and a,a',a"-trifluoro4-toluic acid) show only very weak absorptions in this region.18 Bands in the fingerprint region below 1600 cm-l can be difficultto assign definitively;however they can sometimes offer supporting evidence for structural assignmentsbased on other features. We observed bands in the 1400-1480 cm-l range in all of our spectra; this band has been assigned for carboxylicacids as an in-plane O-H. bend coupled with some C-OH stretching c h a r a ~ t e r . ~Ali>~J~ phatic compounds can also exhibit a symmetric CH2 bending vibration in this range.5*7*19 The 4-mercaptobenzoic acid monolayer spectrum exhibited a band at 1364
(18)( a )The Aldrich Library of FT-IR Spectra, Editions I & 11; The Aldrich Chemical Company: Milwaukee, WI, 1985. (b) The Aldrich Library ofFT-IR Spectra, Edition 111;The Aldrich Chemical Company: Milwaukee, WI,1989.
(19)(a) Roeges, N . G . A Guide t o the Complete Interpretation of Infrared Spectra of Organic Structures; John Wiley: Chichester, 1994. (b)Varsanyi,G.Assignments for Vibrational Spectra of Seven Hundred Benzene Derivatives; John Wiley: New York, 1974.
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1854 Langmuir, Vol. 11, No. 6, 1995
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Table 1. Spectral Features, Peak Positions, and Peak Assignments for Spectra of 16-MercaptohexadecanoicAcid and 4-MercaptobenzoicAcid on Gold and in KBr Glass peak position (wavenumbers) 16-mercaptohexadecanoic acid assignment
monolayer
KBr glass
0-H str CH2 str, sym CH2 str, asym C-H str, arom C=O str C=C str, arom O-H...O bend/C-OH s t P 0 - H bend* C-OH str/O-H..*O bendC C-OH stld 0 - H oop bend C-H ip bend, arom
4-mercaptobenzoic acid monolayer
Kl3r glass
3579 2849 2918
2850 2918
1717,1739
1696
1400-1480
1409, 1473
1272
1150-1350
1749 1480,1589 1400- 1430 1364
3066,3086 1678 1495,1594 1402,1425 1250- 1350
1189 917 1015,1093,1173
927 1017,1097, 1180
a Bands for carboxylic acids in the range 1340-1450 cm-' have been assigned as OH** -0in-plane bending vibrations coupled with some C-OH stretch c h a r a ~ t e r . ~Bands ~ ~ J ~in the range of 1470 cm-l have also been assigned to symmetric CH2 bends in aliphatic c h a i n ~ . ~ , ~ J ~ In free acids, this band is thought to be largely 0-H bend in character, and it occurs a t approximately 50 cm-l lower wavenumber than in aggregated acids.lg Bands for carboxylic acids in the range 1170-1330 cm-l have been assigned as C-OH stretching vibrations with some in-plane 0-He. e 0 bending character coupled in.19 In free acids, this band is thought to be largely C-OH stretch in character (with some 0-H bend added), and it occurs approximately 100 cm-l lower than in aggregated acids.lg
*
cm-l that was absent in all our other spectra; we assign this band as an 0-H bending vibration for a free acid, having noted that free carboxylic acids in the vapor phase typically exhibit an 0-H bending vibration at approximately 50 cm-l lower wavenumber than for acids in condensed phases.lg The band at 1272 cm-l in the spectrum of the 16-mercaptohexadecanoicacid monolayer has been assigned as a C-OH stretching vibration; the absence of a band in this range in the 4-mercaptobenzoic acid spectrum is again consistent with reports that this band occurs approximately 100 cm-l lower in free acids than in aggregated acids.lg We tentatively assign the band at 1189 cm-l in the 4-mercaptobenzoic acid monolayer spectrum to this vibration and the bands at 1015, 1093, and 1173 cm-l to in-plane aromatic C-H bending vibrations. Taken together, these assignments strongly support our postulate that acid groups in the 4-mercaptobenzoic acid monolayer are free ofthe aggregationthat is prevalent in monolayers of alkane-based acids. The fact that acid groups in the aromatic monolayers do not associate with each other suggests not only that the monolayers are conformationally rigid but also that the adsorbate molecules are oriented so as to hold the acid groups apart. The most sensible surface structure that accomplishes this is one in which 4-mercaptobenzoic acid molecules are held to the surface via a gold-sulfur bond with the principal axis of the molecule oriented normal to the surface (or nearly so) and with acid groups oriented away from the surface. There have in fact been independent suggestions that chemisorbed thioaromatic molecules adopt such a surface structure. Hubbard and co-workers and Soriaga and co-workers postulated from surface coverage data that many aromatic sulfur compounds adopt a vertical orientation on noble metal surfaces, including platinum, silver, and gold,20-22and Carron and Hurley postulated from surface-enhanced
Raman spectral data that thiophenol on gold is oriented with the principal axis tilted approximately 14"away from the surface Our spectral data do not yet allow us to make quantitative statements about adsorbate orientation, except to say that the acid groups must be oriented such that hydrogen bonding is disallowed and the transition dipoles of the 0-H and C=O stretches project at least some intensity along the surface normal. Work in progress with other aromatic adsorbates substituted with functional groups having different vibrational signatures with different normal modes and transition dipole vectors should lend further insight into this important aspect of these surface layers. In conclusion, monolayers of 4-mercaptobenzoic acid on gold exhibit infrared spectral features that are characteristic offree,unassociated acid groups and inconsistent with hydrogen-bonded andor dimerized acid groups. This behavior is in contrast with that of aliphatic monolayers, which presumably are flexible enough to adopt the energetically favorable hydrogen-bonded geometry for terminal acid groups. It seems likely that this structural feature of the aromatic monolayer will have consequences for the chemistry of the acid groups at monolayer-vapor and monolayer-solution interfaces.
Acknowledgment. Financial support from the National Science Foundation (Grant CHE 9216361) is gratefully acknowledged. LA941027X (20) Stern, D. A,; Wellner, E.; Salaita, G. N.; Laguren-Davidson,L.; Lu, F.; Batina, N.; Frank, D. G.; Zapien, D. C.; Walton, N.; Hubbard, A. T. J.Am. Chem. SOC.1988,110,4885. (21) Bravo, B. G.;Michelhaugh, S. L.; Soriaga, M. P. J. EZectroanaZ. Chem. 1988,241,199. (22) Gui, J. Y.; Stern, D. A,; Frank, D. G.; Lu, F.; Zapien, D. C.; Hubbard, A. T. Langmuir 1991,7 , 955. (23) Carron, K. T.; Hurley, L. G. J.Phys. Chem. 1991,95, 9979.
